Cheapest priced copy (Morningtide): $0.35 at Card Kingdom, near mint, their price list as of .
Rivals' Duel 
Sorcery · Uncommon · Morningtide · #99
Choose two target creatures that share no creature types. Those creatures fight each other. (Each deals damage equal to its power to the other.)
Legal inCommanderModernLegacyVintage
Not legal inBrawlStandardPioneerPauper
4 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Morningtide | 99 | Uncommon | $0.35 | $0.49 |
| Planechase 2012 | 51 | Uncommon | $0.35 | — |
| Planechase Anthology | 51 | Uncommon | $0.35 | — |
| The List | PCA-51 | Uncommon | $0.35 | — |
3 rulings from Wizards of the Coast
- 1 April 2008The two creatures may be controlled by the same player.
- 1 April 2008If, by the time Rivals’ Duel resolves, an effect has caused the two target creatures to share a creature type, Rivals’ Duel doesn’t resolve for having no legal targets.
- 1 April 2008If either one of the creatures leaves the battlefield before Rivals’ Duel resolves, no damage is dealt to or by the remaining creature. If both creatures leave the battlefield before Rivals’ Duel resolves, the spell doesn’t resolve for having no legal targets.